<!DOCTYPE html>
<html lang="en">
<head>
    <meta charset="utf-8">
    <title>计算器</title>
    <style>
        #calculator{
            margin: 15px auto;
            width:280px;
            height:400px;
            border: 1px solid lightgray;
            background-color:darkgrey;
            padding:15px;
        }
        .LOGO{
            height:15px;
        }
        .LOGO .name{
            float:left;
            line-height:10px;
        }
        .LOGO .author{
            float:right;
            line-height:10px;
        }
        #Input{
            margin-top:1px;
            margin-right:20px;
            width:219px;
        }
        .screen{
            margin-top:5px;
            width:225px;
            height:40px;
            text-align: right;
            padding-right:10px;
            font-size:20px;
        }
        #keys{
            border:1px solid lightgray;
            height:260px;
            width:219px;
            margin-top:25px;
            padding:8px;
        }
        .footer{
            margin-top:20px;
            height:20px;
        }
        #keys .buttons{
            float:left;
            width: 42px;
            height: 36px;
            text-align:center;
            background-color:lightgray;
            margin: 0 17px 20px 0;
        }
    </style>
</head>
<center>
    <div id="calculator">
        <div class="LOGO">
            <span class="name">简单的计算器</span>
            <span class="author">@申奥</span>
        </div>
        <div id="Input">
            <div class="screen">
                <input type="text" id="screenName" name="screenName" class="screen" value="" onfocus="jsq(this)">
            </div>
        </div>
        <div id="keys">
            <input type="button" id="C" onclick="clearNum()" value="C" class="buttons">
            <input type="button" id="+/-" onclick="jsq(this.id)" value="+/-" class="buttons">
            <input type="button" id="Back" onclick="tuiGe()" value="Back" class="buttons">
            <input type="button" id="/" onclick="jsq(this.id)" value="/" class="buttons" style="margin-right:0px">
            <input type="button" id="7" onclick="jsq(this.id)" value="7" class="buttons">
            <input type="button" id="8" onclick="jsq(this.id)" value="8" class="buttons">
            <input type="button" id="9" onclick="jsq(this.id)" value="9" class="buttons">
            <input type="button" id="*" onclick="jsq(this.id)" value="*" class="buttons" style="margin-right:0px">
            <input type="button" id="4" onclick="jsq(this.id)" value="4" class="buttons">
            <input type="button" id="5" onclick="jsq(this.id)" value="5" class="buttons">
            <input type="button" id="6" onclick="jsq(this.id)" value="6" class="buttons">
            <input type="button" id="-" onclick="jsq(this.id)" value="-" class="buttons" style="margin-right:0px">
            <input type="button" id="1" onclick="jsq(this.id)" value="1" class="buttons">
            <input type="button" id="2" onclick="jsq(this.id)" value="2" class="buttons">
            <input type="button" id="3" onclick="jsq(this.id)" value="3" class="buttons">
            <input type="button" id="+" onclick="jsq(this.id)" value="+" class="buttons" style="margin-right:0px">
            <input type="button" id="0" onclick="jsq(this.id)" value="0" class="buttons">
            <input type="button" id="00" onclick="jsq(this.id)" value="00" class="buttons">
            <input type="button" id="." onclick="jsq(this.id)" value="." class="buttons">
            <input type="button" id="eva" onclick="eva()" value="=" class="buttons" style="margin-right:0px">
        </div>
        <div class="footer">
            <span>欢迎使用JavaScript计算器</span>
        </div>
    </div>
</center>
<script>
    var num = 0;
    function jsq(num) {
        document.getElementById('screenName').value += document.getElementById(num).value;
    }
    function eva() {
        document.getElementById("screenName").value = eval(document.getElementById("screenName").value);
    }
    function clearNum() {
        document.getElementById("screenName").value = null;
        document.getElementById("screenName").focus();
    }
    function tuiGe() {
        var arr = document.getElementById("screenName");
        arr.value = arr.value.substring(0, arr.value.length - 1);
    }
    function onLoad(){
        document.getElementById("screenName").focus();
    }
</script>
</body>
</html>